1. Field of Technology
The present invention relates to a two-stage electrostatic filter (electrostatic precipitator), and more specifically to a two-stage electrostatic filter.
2. Prior Art
Electrostatic filters, also called electrostatic dust separators, are used both in industrial production plants, in which case the electrostatic filters are in the form of large and expensive apparatus, and in apparatus in which air is cleansed for comfort purposes, such as air-conditioning apparatus and other apparatus for use in domestic dwellings, offices and other places of work, schools, hospital care facilities, motor vehicles and other places in which the air can be cleansed with comparatively much smaller apparatus.
In this latter case, in which it is mostly the air present in occupied places or the air entering such places that is to be cleansed, the filters used have hitherto essentially comprised mechanical filters provided with fibre filter cloths, textile or paper-based fibre-filter mats or electrode filter mats.
Electrostatic filters have also been used to a certain extent in this latter case. These electrostatic filters have normally been two-stage electrostatic filters by which is meant electrostatic filters in which the solid or liquid particles, aerosols, which are carried by the airflow and which are to be extracted therefrom are electrically charged in a separate ionization section while the actual separation process takes place in a capacitor separator positioned downstream of the ionization section. The present description is concerned with two-stage electrostatic filters, unless stated otherwise.
Mechanical air filters almost exclusively use disposable or exchangeable filter elements. Thus, those parts of the filter which primarily capture the separated material and which are therefore the filter components that are most subjected to dirt and clogging constitute units which can be readily exchanged. These elements, or units, are used until they can no longer fulfil their intended function in a satisfactory manner and are then replaced with a new unit and scrapped.
Hitherto, disposable units have not been used in electrostatic filters; at most, the capacitor separators typically comprised of aluminium plates and high-grade insulating material have been given the form of cassettes which can be removed readily from the filter apparatus for cleaning purposes. The task of cleaning these cassettes, however, is both time-consuming and expensive and can result in the spreading of unhealthy dust. Electrostatic filters are also expensive to run.
Because of these high running costs, electrostatic filters have not been used to an extent which corresponds to the important advantages that electrostatic filters afford over mechanical filters.
Another contributory cause lies in the fact that present-day electrostatic filters have a complicated and expensive construction due to the use of high voltages and the safety requirements associated therewith, such as the requirement of touch-safe designs and the use of high-grade materials, for instance for the insulators. A further contributory cause lies in the necessity of using high corona current intensities in order to avoid poor separation efficiency, which in turn results in a substantial generation of irritating odourants (ozone) in the chemically highly active plasma layer adjacent the corona electrode, or limits the cleansing capacity of the apparatus.
Furthermore, in conventional electrostatic filters, dust collected on the electrodes of the capacitor separator often causes sparkover between the electrodes, resulting in problems when using the filter in sensitive environments and danger of complete loss of the separating function.
Among the advantages which electrostatic filters afford in comparison with mechanical filters is that despite causing a very small pressure drop in the gas flow to be cleansed, electrostatic filters have the ability to separate extremely small particles from the gas flow; typical respirable particles have a diameter of about 0.3 .mu.m. Mechanical filters always have a considerable pressure drop. In particular, in the case of filters that are constructed to separate respirable particles from the gas flow, the pressure drop across the actual filter part (the filter element) is extremely high. This high pressure drop necessitates the use of noisy and power-demanding fans for transporting the gas through the filter.